1. Field of the Invention
The present invention relates to a double fed synchronous generator motor which is connected to an electric power system at a stator side and to a frequency-variable power converter at a rotor side, coupled to a reversible pump-turbine and operated while switched to an electric power generating operation mode or a pump-up operation mode.
2. Description of the Related Art
A double fed synchronous generator motor can be generally operated at fixed upper and lower speeds with the synchronous speed at the center, that is, within the range of a variable speed width. Furthermore, as disclosed in JP-A-61-240829 (page 4, FIGS. 2 to 5), optimum variable speed widths are set to a pumping operation and a power generating operation respectively in a variable speed pumped-storage power generating system using a double fed synchronous generator motor. The variable speed width is fixed to a value set for any power generation output or pumping input, and it is practically applied as a fixed value at all times irrespective of the operation state.
It is generally known that the input in the pumping operation of a reversible pump-turbine varies hardly even when the guide vane opening is changed, and it varies substantially in proportion to 2.5 to 3 powers of the rotating speed. A variable speed pumped-storage power generating system is introduced to implement AFC operation even during pumping operation, and the variable speed width of the double fed synchronous generator motor applied to the variable speed pumped-storage power generating system is determined by the AFC capacity in the pumping operation.
For example, a pump input of 300 MW is required at the synchronous speed, a reversible pump-turbine whose input is proportional to 3 powers of the rotating speed is assumed, and if the variable speed width is set to the range of ±10% of the synchronous speed with the synchronous speed set at the center of the range, the AFC capacity of 300 MW×(1.13−0.93)=399.3 MW−218.7 MW=180.6 MW can be secured.
In order to operate the double fed synchronous generator motor in the range of ±10% of the rated speed, it is necessary for the variable frequency power converter to apply a voltage having a frequency in the range of ±10% of the rated frequency (called as slip frequency) to the winding of the rotor of the double fed synchronous generator motor. In the case of a 50 Hz system, an exciting voltage of ±5 Hz frequency must be output.
Accordingly, the required maximum output voltage of the variable frequency power converter corresponds to the voltage obtained by multiplying the exciting current required to perform the pumping operation of 399.3 MW by the impedance of the winding of the rotor. This is because the magnitude of the exciting current is maximum for the input of 399.3 MW and the impedance of the rotor is substantially proportional to the applied frequency, so that the maximum voltage is obtained at +5 Hz.
Since the rotating speed in the operation of the minimum pump input of 218.7 MW is equal to −5%, the exciting frequency of the rotor is also the negative maximum value, and the impedance of the winding of the rotor is also maximum. However, the exciting current is small and thus there is no problem if the output voltage of the variable frequency power converter is small. Accordingly, it does not become a factor for determining the required maximum output voltage of the exciting device.
In general, the rotating speed of the double fed synchronous generator motor is operated within the range of a variable speed width adopted to secure a required AFC capacity. That is, the variable speed width is fixed to the value of a variable speed width in which the upper and lower limit values of the rotating speed with respect to the synchronous speed are fixed in the overall area of the water turbine output irrespective of the water turbine output of the reversible pump-turbine.
Accordingly, it is known from the performance characteristic of the reversible pump-turbine that the water turbine efficiency of the reversible pump-turbine is higher as the rotating speed is lower, and thus it is preferable to operate the reversible pump-turbine at a rotating speed which is as low as possible. However, the rotating speed is restricted by the lower limit of the variable speed width, and thus in many cases the reversible pump-turbine is operated while the speed is fixed to the lower limit speed of the variable speed width.
On the other hand, when the electric output of the water turbine output of the reversible pump-turbine, that is, the double fed synchronous generator motor is low, the magnitude of the exciting current may be small, and thus there is a margin in the exciting voltage. Therefore, the exciting voltage can be increased by further lowering the rotating speed. That is, when the output is low, the rotating speed can be further reduced.
From this viewpoint, there is a merit that if the operation is carried out while the lower limit of the variable speed width is reduced when the output in the power generating operation is small, the operation could be carried out with a high water turbine efficiency. In the pumping operation, the lower limit of the variable speed width can be likewise reduced in the case of a low pumping input operation, that is, when the rotating speed is small, and thus the operation can be performed with a smaller pumping input. In other words, there is a merit that the AFC capacity can be further increased.
Accordingly, the operation at a fixed variable speed width over the whole area of the water turbine output of the reversible pump-turbine means that an opportunity of enhancing the efficiency at a low area of the water turbine output is missed and also an opportunity of increasing the AFC amount is missed in the case of the pumping operation.
In other words, there is a challenge of enhancing the power generation efficiency in the low rotating speed area and the pumping AFC amount in the water turbine operation or in the pumping operation in the variable speed pumped-storage power generating system based on the double fed synchronous generator motor.